A technique for enhancing corrosion resistance and wearing resistance by coating a surface of a metal material by a submerged electrical discharge processing method is heretofore known. An example of the technique is as follows. For example, a method has been disclosed in which a pulsed electrical discharge is performed in working fluid with an electrode obtained by compression molding a mixture of fine particles of WC (tungsten carbide) and Co (cobalt) to deposit the electrode material on a workpiece, and thereafter re-melt discharge processing is performed with another electrode (for example, a copper electrode or a graphite electrode) to obtain even higher hardness and enhanced adhesion (see, for example, Patent Document 1). In Patent Document 1, electrical discharge processing is performed on a workpiece (a base material S50C) in working fluid using a WC—Co mixed green compact electrode to deposit the WC—Co on the workpiece (primary processing), and subsequently, re-melt processing (secondary processing) is performed using an electrode that is not relatively susceptible to wearing, such as a copper electrode. As a result, although the deposited structure has a hardness (Vickers hardness Hv) of about Hv=1410 with a considerable amount of voids after the primary processing, the voids of the coating layer disappear with enhanced hardness of Hv=1750 by the secondary processing of the re-melt processing. With this method, a coating layer having a great hardness with enhanced adhesion is obtained for steel as the workpiece.
However, with the above method, it is not possible to form a coating layer having robust adhesion on a surface of a sintered material, such as cemented carbide, as the workpiece. In regard to this point, researches performed by present inventors have shown that a robust hard coating can be formed on a metal surface of a workpiece without performing the re-melt processing by generating electrical discharge between an electrode and the workpiece, using a material such as Ti (titanium), which forms hard carbide, as the electrode. This is because the electrode material worn by the electrical discharge and C (carbon) in the working fluid react with each other, and as a result, TiC (titanium carbide) is produced.
Furthermore, a technique has been disclosed in which a hard coating film can be formed with good adhesion faster than a case of using materials such as Ti and the like by generating electrical discharge between a workpiece and an electrode by using a green compact obtained from a metal hydride such as TiH2 (titanium hydride) as the electrode (see, for example, Patent Document 2). In addition, another technique has been disclosed in which a hard coating film having properties of hardness, wearing resistance and the like is formed in a rapid manner by generating electrical discharge between a workpiece and an electrode by using a green compact obtained from a mixture of the metal hydride such as TiH2 and the like and metal or ceramic.
Further, still another technique has been disclosed in which an even stronger surface treatment electrode can be manufactured by pre-sintering (see, for example, Patent Document 3). That is, when manufacturing an electrical-discharge surface-treatment electrode made of a mixed powder of the WC powder and the Co powder, although a green compact can be obtained by simply compression molding the mixture of the WC powder and the Co powder, if it is compression molding after mixing wax, the molding property of the green compact is enhanced. In this case, because the wax is a dielectric material, when a considerable amount of wax remains in the electrode, an electrical resistance of the electrode increases, resulting in degradation of the discharge property. Therefore, the green compact electrode is heated in a vacuum furnace to remove the wax.
At this time, when the heating temperature is too low, the wax is not completely removed, and when the heating temperature is too high, the wax turns into soot, which degrades purity of the electrode. Therefore, the heating temperature needs to be maintained between a temperature at which the wax is melted or higher and a temperature lower than the wax is dissolved to turn into the soot. By heating the green compact in the vacuum furnace using a high-frequency coil and the like, it is baked until it has a hardness of about, for example, a chalk, and giving a strength to resist a machining process, paying attention not to harden excessively. This state is referred to as a pre-sintering state. In this case, although a bonding is progressed in an area of contact between carbides in the green compact, the bonding is weak because the sintering temperature is relatively low not reaching regular sintering. It has been proved that a dense and homogeneous coating film can be formed by performing electrical discharge surface treatment using an electrode formed in the above manner.
However, the conventional techniques described above need a further improvement because, although they are featured in points of the hardness and adhesion of the coating film, the wearing resistance and the swiftness of forming the coating film, and the density and homogeneity of the coating film, all the techniques fail to obtain sufficient thickness of the coating film.
A common technique for obtaining a thick coating film is so-called welding/thermal spraying. The welding (which means cladding by welding in this context) is a method of depositing a material of a welding electrode on a workpiece by melting the welding electrode with electrical discharge between them. The thermal spraying is a method of forming a coating film on a workpiece by melting a metal material and spraying it onto the workpiece. Because both methods are done by a human, requiring a skilled workforce, it is difficult to make the work a line operation, having a disadvantage of high manufacturing cost. Particularly, because the welding is a method in which the heat is applied to the workpiece in a concentrated manner, it causes a problem that a weld cracking occurs and a yield rate is low when processing a thin or fragile material.
Although it is described that a thick coating film of about 3 millimeters has been obtained using a WC—Co (9:1) electrode in electrical discharge surface treatment in a document based on the researches conducted by the present inventors (see Nonpatent Literature 1), it does not reach a level that can be practically applicable because of problems that the reproduction of the coating film is difficult because the formation of the coating film is unstable, the coating film is brittle having a considerable amount of voids although it is seemingly dense with a metallic gloss, and the coating film is so weak that it is removed when it is scratched hard with a metal piece and the like.
Subsequent researches have shown that a dense and thick coating film can be formed by electrical discharge surface treatment by mixing a predetermined amount or more of a metal material that does not form carbide or hardly forms carbide in the electrode material. That is, a thick coating film can be formed by increasing materials such as Co (cobalt), Ni (nickel), and Fe (iron) in the electrode material, reaching a level that can be practically applicable as a thick coating film (see Patent Document 4).    Patent Document 1: Japanese Patent Application Laid-open No. H5-148615    Patent Document 2: Japanese Patent Application Laid-open No. H9-192937    Patent Document 3: Japanese Patent No. 3227454    Patent Document 4: International Publication No. WO 04/011696 pamphlet    Nonpatent Literature 1: Technique of “Formation of Thick Layer by Electrical Discharge Coating (EDC)” Akihiro GOTO et al., (1999), THE NIKKAN KOGYO SHIMBUN, LTD.